The present invention relates to a method and apparatus for eliminating slug flow in multiphase pipelines and process equipment and for separating liquid phases from the gas phase in such a mixture. More particularly, the present invention relates to a method and apparatus for eliminating slug flow in a flow line containing petroleum liquids and natural gas and to a method and apparatus for separating the liquid phase of such a mixture from the gas phase thereof.
When liquid/gas mixtures move through a horizontal or inclined pipeline, the flow can take several configurations. At low liquid and low gas flow rates, the flow is stratified such that the gas phase flows above the liquid phase. At higher flow rates, waves form at the gas/liquid interface. If the gas phase flow rates are high enough, the waves can grow to the point where they fill the cross-section of the pipeline, blocking the gas flow. Such waves are termed slugs. Inasmuch as the flow rate of the gas phase is much higher than that of the liquid phase, the liquid slugs are accelerated by the gas phase to approximately the same velocity as the gas phase. The slugs thus travel at a much higher flow rate than the non-slug portion of the liquid phase. As a result, the piping and related equipment downstream of the slugs experience intermittent surges and subsequent impact from the slugs. As can be appreciated, such surging places tremendous stresses on the piping and process equipment.
When crude oil flows from wells, it is usually accompanied by natural gas, sand, water, and other components which produce a mixed phase flow. The oil/gas mixture often flows at production rates which place it in the slug flow regime. In addition, wells which have been partially depleted are generally subjected to enhanced recovery techniques to extract the remaining oil therefrom. Such techniques employ the injection of steam, gas, water, or chemicals to force the oil the oil bearing formation and thereby exacerbate the inherent slug flow problems associated with petroleum extraction.
It is well known to those skilled in the art that the stresses placed on oil pipelines by slug flow is a leading cause of their failure. Such stresses not only damage the pipelines and related equipment, but vibrations, abrupt pressure changes, and flow characteristics lead to corrosion in the piping and downstream equipment. To minimize such deleterious effects, crude oil is pumped at flow rates which are lower than the maximum potential output of the wells. However, this is undesirable from an economic standpoint since profitability of each well is directly proportional to the flow rate of petroleum therefrom. Thus, it is seen that there is a strong need to eliminate or control slug flow in petroleum pipelines.
Current solutions to the problem of slug flow provide large, complicated structures which are very expensive to manufacture and install. For example, Huntley, UK Patent No. 2,153,251, discloses a submersible liquid/gas separator apparatus with at least one hollow elongated separator pressure vessel into which a liquid/gas mixture is received. The separator vessel further includes two gas outlets and two liquid outlets. The liquid outlets are connected to a series of hollow elongated liquid accumulator pressure vessels which are rigidly interconnected and are positioned side-by-side in parallel with one another. The accumulator vessels are in fluid communication with one another and provide storage for collected oil.
Asheim, WO 87/01759, discloses a device for gathering liquid slugs in a pipeline. The pipeline leads to a large pipe loop on the seabed which has enough volume to gather all of the liquid from a slug. The loop leads to a riser which carries the liquid from the slug up to a separation tank on a platform.
Such prior art slug flow eliminators have several inherent drawbacks. They are large, complicated, expensive, and difficult to install and remove. Some commercially available slug flow eliminators, for example, weigh 60 tons, cost approximately $3-4 million, and measure 16 feet in height, 16 feet in width, and 23 feet in length. Such prior art slug flow eliminators are unsuitable for enhanced recovery of oil from partially depleted fields and for use in marginal fields.
In order for enhanced recovery from partially depleted fields to be profitable, the capital costs must kept to a minimum due to the relatively small amounts of oil that remain. As discussed above, enhanced recovery techniques result in severe slug flow problems. Although slug flow eliminators would make extraction of oil from partially depleted fields much easier, the high cost of current devices makes their use economically unfeasible.
Marginal fields are undepleted areas where small volumes of oil are present beneath the surface. In order to remove the oil in a cost effective manner, the capital outlay for such removal must be kept small. This can best be achieved by utilizing existing gathering stations and/or re-useable equipment. Most marginal fields are located great distances away from existing gathering stations. There are many different types of gathering stations. Some are located beneath the surface of the ocean, some on the surface of the ocean such as a drilling platform, and some are located on land. As those skilled in the art will appreciate, pumping oil over significant distances leads to severe slug flow problems. One or more slug flow eliminators are needed for each such pipeline in these circumstances. However, due to the expense of traditional slug flow eliminators and the relatively small amount of oil which can be extracted from each well in a marginal field, it is simply not cost effective to extract this oil.
An additional problem is that conventional slug flow eliminators are not re-useable. The pipelines to which they are to be attached must be significantly altered in order to accommodate their complicated configurations. It is not practical to make such extensive modifications repeatedly in order to reuse the slug flow eliminators. In addition, the size and weight of the slug flow eliminators of the prior art make them difficult to handle. Installation is a major undertaking.
Many current slug flow eliminators cause frothing and emulsions to occur in liquid/gas mixtures because the mixtures are brought to an abrupt halt inside the eliminators. The resultant turbulence produces liquid/gas emulsions or foam. Emulsion-breaking chemicals and mist eliminators must be used with such devices.
In order to remove scale and other types of buildup from the inside of petroleum pipelines, tight-fitting objects, referred to in the art as "pigs", are periodically forced through the pipeline. As is known by those skilled in the art, it is advantageous to be able to retrieve pigs from the pipeline at the slug flow eliminator. Due to the complicated structure of most slug flow eliminators, however, the retrieval of pigs therefrom is impossible.
A second area in which improvement over the prior art is needed concerns the separation of liquid/gas mixtures at ocean-based oil gathering stations. When petroleum is pumped from an undersea well to a gathering station, the gas phase must be separated from the liquid phase so that both phases can be processed separately. Currently, large separator tanks with vertical baffles are used for this purpose. As is known in the art, oceanic gathering station space is at a premium. The separator tanks currently in use, typically measuring 20 feet by 80 feet, take up large amounts of such space. In addition, such tanks are not equipped to handle severe slug flow. The vertical baffles are frequently destroyed by slugs of liquid impinging upon them and must be replaced. What is needed in this regard is a compact liquid/gas separator which is equipped to handle slug flow.
Accordingly, a need exists in the art for an in-line slug flow eliminator which is inexpensive, easy to maneuver, and re-useable, which does not produce foaming or an emulsion, and which allows the retrieval of pigs therefrom. In addition, a need exists for a liquid/gas separator for use with gathering stations which is both compact and able to eliminate slug flow.